Direct Synthesis of Organonitrogen Compounds from Dinitrogen Using Transition Metal Complexes: Leap from Stoichiometric Reactions to Catalytic Reactions

Organonitrogen compounds are of vital importance to our lives. For almost all organonitrogen compounds, ammonia is only the feedstock as a nitrogen source. Ammonia is produced from dinitrogen (N2) and dihydrogen (H2) by the Haber‐Bosch process, which consumes an enormous amount of energy. As an alternative method to convert N2 into organonitrogen compounds under milder conditions, various stoichiometric reactions with transition metal complexes have been explored for the last 50 years. However, the field of catalytic formation of organonitrogen compounds directly from N2 with transition metal complexes is still in its infancy. This short review summarizes strategies to achieve the direct synthesis of organonitrogen compounds from N2 under mild reaction conditions using transition metal complexes ranging from stoichiometric reactions to catalytic reactions.


Introduction
Dinitrogen (N 2 ), the main constituent of air, can be regarded as an easily accessible and unlimited precursor of nitrogencontaining compounds.However, we cannot use N 2 as a nitrogen precursor due to inertness of N 2 derived from its extremely strong triple bond (941 kJ/mol).Currently, the industrial use of N 2 totally relies on the Haber-Bosch process, which uses iron-based heterogeneous catalysts to produce ammonia (NH 3 ) from N 2 and hydrogen (H 2 ) under harsh reaction conditions (high temperature and pressure such as 400-500 °C, 150-250 atm). [1,2]Subsequently, a significant amount of NH 3 produced by the Haber-Bosh process is used as a feedstock for value-added nitrogen-containing compounds.This industrial process, described as "making bread from air," [3] is also known as an energy-intensive process that accounts for more than 1 % of the total energy consumed by human beings and emits an enormous amount of CO 2 when H 2 is produced by steam reforming. [4]s an alternative method to convert N 2 under milder conditions, the use of transition metal complexes has been explored enthusiastically.The discovery of coordinated N 2 on transition metal complexes derived from hydrazine [5] or N 2 gas [6] demonstrated the potential of transition metal complexes to capture and activate N 2 .Since then, stoichiometric or catalytic formation of nitrogen-hydrogen bonds to afford NH 3 using transition metal complexes with appropriate proton sources and reductants has been developed, [7] and turnover number of 60,000 has been achieved quite recently by the state-of-the-art catalyst. [8]here has been also high motivation to directly convert N 2 into nitrogen-containing compounds other than NH 3 by forming nitrogen-non-hydrogen bonds, instead of nitrogen-hydrogen bonds, on transition metal complexes under mild reaction conditions, considering that a large part of produced NH 3 by the Haber-Bosch process is finally converted to nitrogencontaining compounds.Among them, the formation of nitrogen-carbon bonds to produce organonitrogen compounds is the most attractive approach because of their diversity and vital importance to our lives as biomolecules, medicines, and chemical fibers.In addition, development of such reactions contributes to reduce the number of reaction steps and to save a large amount of energy consumption upon NH 3 formation.To achieve this goal, the formation of nitrogen-carbon bonds at N 2 ligands or nitride ligands derived from N 2 on transition metal complexes has been intensively investigated as stoichiometric reactions.However, compared to the successful examples of catalytic formation of nitrogen-hydrogen bonds from N 2 to produce NH 3 , [9] catalytic formation of nitrogen-carbon bonds to produce organonitrogen compounds is highly elusive and has not been achieved until recently. [10][13] First, early examples of the formation of nitrogen-carbon bonds from metal-N 2 and metal-nitride complexes are summarized.Then, synthetic cycles established based on these stoichiometric reactions are introduced.Finally, we demonstrate how to extend the synthetic cycles based on stoichiometric reactions to catalytic reaction to synthesize nitrogen-carbon bonds containing compound directly from N 2 .

Stoichiometric Formation of Nitrogen-Carbon Bonds on Dinitrogen/Hydrazide Ligands
The first successful example of the formation of nitrogencarbon bonds on dinitrogen (N 2 ) ligand was reported by Chatt and co-workers in 1972 (Scheme 1). [14,15]A tungsten N 2 complex bearing 1,2-bis(diphenylphosphino)ethane (dppe) ligands trans-[W(N 2 ) 2 (dppe) 2 ] (1) reacted with acyl chlorides as carboncentered electrophiles to form the corresponding cationic acylhydrazide complex [W(NNHCOR)(dppe) 2 Cl]Cl, where a proton was supplied from a trace amount of water in the system.The proton was removed by addition of Et with HCl regenerated W-(NNHCOR)(dppe) 2 Cl]Cl.Chatt's and George's groups independently found the alkylation of molybdenum-or tungsten-N 2 complexes with alkyl halides under light irradiation to afford Nalkylated complexes. [16,17]N,N-Dialkylhydrazide complexes of molybdenum or tungsten thus formed gave corresponding secondary alkyl amines in good yields upon treatment with LiAlH 4 . [18][21] Based on the findings, Hidai and coworkers reported a synthetic cycle starting from the tungsten-N 2 complex to release the corresponding pyrroles (Scheme 3). [22,23]Protonation of 1 with HBF 4 gave the cationic hydrazide complex [W(NNH 2 )(dppe) 2 F]BF 4 (2).The reaction of 2 with 2,5-dimethoxyfurane afforded the cationic pyrrolimide complex (3).Treatment of 3 with LiAlH 4 followed by quenching with MeOH gave the corresponding tetrahydride complex (4), pyrrole, and ammonia together with a small amount of N-aminopyrrole.Finally, 4 can be converted to the starting N 2 complex 1 by light irradiation under N 2 atmosphere.
N-arylation of coordinated N 2 with aryl halides to give aryldiazenide complexes via S N Ar-type reaction was reported by Hidai and coworkers. [24]A molybdenum-N 2 complex bearing tetradentate crown thioether ligand (5), which is more nucleophilic than its phosphine analogues, reacted with an aryl halide to give the corresponding aryldiazenide complex (Scheme 4a).On the other hand, 5 was further converted to the corresponding hydrazide complex ( 6 Scheme 1. Acylation of molybdenum N 2 complex with acyl chloride. [15]21] room temperature.As another example, Hidai and coworkers adopted the "bimetallic approach", where an anionic tungsten-N 2 complex [ n Bu 4 N][W(N=C=S)(N 2 )(dpe) 2 ] (7) reacted with [Cr(p-FC 6 H 4 CO 2 Me)(CO) 3 ] to give the corresponding aryldiazenide complex (Scheme 4b). [25]The activation of arene with Cr(CO) 3 moiety is critical to achieve this unique reactivity, since 7 failed to react with non-coordinated p-FC 6 H 4 CO 2 Me.
Peters and coworkers reported the formation of nitrogencarbon bonds on terminal N 2 ligand of late transition metal complexes such as iron and cobalt complexes.Reactions of anionic iron-and cobalt-N 2 complexes bearing tridentate phosphine ligands with MeOTs (OTs = OSO 2 (4-CH 3 -C 6 H 4 ) at room temperature afforded the corresponding monomethyldiazenide complexes (Scheme 5a). [26]The iron-N 2 complex bearing a tetradentate ligand allowed the further reaction with MeOTf (OTf = OSO 2 CF 3 ) to afford the corresponding dimethyl hydrazide complex (Scheme 5b). [27]uite recently, Xi and coworkers reported the stepwise functionalization of N 2 ligand on an anionic chromium complex bearing Cp* ligand K[Cp*Cr(depe)N 2 ] (Cp* = η 5 -C 5 Me 5 , depe = 1,2-bis(diethylphosphino)ethane) with MeOTf to give a cationic dimethyl hydrazide complex [Cp*Cr(depe)(= NNMe 2 )]OTf via a monomethyl diazenide complex [Cp*Cr(depe)(N=NMe)]. [28] In contrast to the addition of carbon-centered electrophiles to terminal N 2 ligand, Sellman and coworkers reported the reaction of a manganese-N 2 complex bearing a Cp ligand [Cp(CO) 2 Mn(N 2 )] (Cp = η 5 -C 5 H 5 ) with phenyl lithium (PhLi) as a carbon-centered nucleophile to form the anionic diazenide complex Li[Cp(CO) 2 Mn(NPh=N)]. [29]However, a recent study showed that the proposed reaction is unlikely to occur, and suggested that PhLi reacts with one of the CO ligands to provide an anionic acylcarbonyl dinitrogen complex Li[Cp-(CO)(N 2 )MnCOPh], then it decomposed to give an anionic phenyl complex Li[Cp(CO) 2 MnPh], which was misassigned in the original literature as the anionic diazenide complex with nitrogen-carbon bond. [30]olland and coworkers took a different approach to achieve the synthetic cycle for an aniline derivative from N 2 and arenes mediated by diketiminate-supported iron complexes, involving CÀ H activation of an arene as well as migration of an aryl group from the iron to the α-position of N 2 ligand as key steps (Scheme 6). [31,32]s well as reactions of terminal N 2 ligands with carboncentered reactants, the formation of nitrogen-carbon bonds on bridged N 2 ligands has been investigated.While N 2 -bridged dihafnium [33] or discandium [34] complexes were reported to undergo alkylation with alkyl (pseudo) halides to give corresponding N-functionalized complexes, cycloaddition or insertion reactions of unsaturated organic molecules to bridged N 2 ligands on early transition metal complexes have been well investigated.Such reactivity offers synthetic methods complementary to that based on the addition of carbon-centered electrophiles to terminal N 2 ligand.Fryzuk and coworkers reported that the reaction of a N 2 -bridged dizirconium complex bearing a diamido-bis(phosphine) ligand ( 8) with arylacetylene to form nitrogen-carbon bonds. [35]In this reaction, the formation of zircona-aza-cyclobutene intermediate through cycloaddition of alkyne across a zirconium-nitrogen bond is followed by cleavage of the zirconium-carbon bond by protonation with the second terminal alkyne to give the corresponding styryl-hydrazide complex (9, Scheme 7).Unfortu-Scheme 3. Synthetic cycle to afford pyrrole and ammonia based on the reaction of hydrazide complex with 2,5-dimethoxyfurane. [22,23] Scheme 4. N-arylation of terminal N 2 ligand with aryl halides via S N Ar-type reaction. [24,25]heme 5. Formation of nitrogen-carbon bonds on terminal N 2 ligand of late transition metal complexes. [26,27]ately, no release of organonitrogen compounds from complex 9 was observed at all.

Stoichiometric Formation of Nitrogen-Carbon Bonds on Nitride Ligands via Cleavage of N�N Bond of Dinitrogen
As mentioned in the previous section, reactions of N 2 ligand with carbon-centered electrophiles typically result in formation of hydrazine derivatives retaining their nitrogen-nitrogen bond.Thus, development of other methods is desired to synthesize various types of organonitrogen compounds such as amine, amide, nitriles, and heterocycles without retaining nitrogennitrogen bond.The alternative method to directly synthesize organonitrogen compounds from N 2 is the use of terminal or bridged nitride ligands generated via cleavage of N�N bond of N 2 . [40][43] For example, Volpin and coworkers found the reaction of the titanium complex [Cp 2 TiCl 2 ] with phenyl lithium at room temperature under the atmospheric to high pressure of N 2 to afford aniline in low yield. [41]However, these nitrogenous titanium complexes are ill-defined, and the reaction mechanisms of these reactions were unclear.Recently, Scheme 6. a) Binding and functionalization of N 2 using diketiminatesupported iron complexes and b) Synthetic cycle for conversion of N 2 and benzene to aniline derivatives. [32]heme 7. Functionalization of bridged N 2 ligand on dinuclear zirconium complex with terminal alkynes. [35]heme 8. Cycloaddition of a) phenyl isocyanate (PhNCO) and b) CO 2 to bridged N 2 ligand on dihafnium complex. [36,38]he preparation of aniline derivatives from lithium nitride as a nitrogen source was reported by Shi and coworkers. [44]n 1995, Cummins and coworkers reported the first successful example of N�N bond cleavage using transition metal complexes under mild reaction conditions to give welldefined nitride complexes (Scheme 9).The molybdenum complex bearing a tris(amide) ligand [Mo(N( t Bu)(3,5-Xyl)) 3 ] (10, 3,5-Xyl = 3,5-dimethylphenyl) reacted with N 2 to give the corresponding N 2 -bridged dinuclear molybdenum complex at À 35 °C as a reactive intermediate, then cleavage of bridged N 2 ligand took place at room temperature to give the corresponding terminal nitride complex [Mo(N( t Bu)(3,5-Xyl)) 3 ] (11). [45]ummins and coworkers also reported the cleavage of N 2 ligand on the heterodinuclear niobium/molybdenum complex (12), which was generated from the corresponding molybdenum-nitride complex ( 13) and anionic niobium-nitride complex [Nb(N)(N( i Pr)(3,5-Xyl)) 3 ] ( 14). [46]Since these milestone works, the formation of terminal nitride complexes via direct cleavage of N 2 ligand with various transition metal complexes has been intensively studied.Other than the example of cleavage of N�N bond using molybdenum complexes, [47,48] N�N bond cleavage reactions using tungsten [49] or rhenium [50,51] complexes have been developed.The N�N bond cleavage reactions are applied to the stoichiometric and catalytic synthesis of ammonia as reviewed comprehensively. [9,40]In 2017, Nishibayashi and coworkers reported the catalytic ammonia formation under mild reaction conditions using molybdenum triiodide complex (15)  via direct cleavage of bridged N 2 ligand and subsequent nitrogenÀ hydrogen bond formation (vide infra). [52]hese findings provide the opportunity to combine nitrogen fixation chemistry and nitrogen atom transfer chemistry in addition to the ammonia synthesis via nitrogenÀ hydrogen bond formation.Therefore, reactivity of the nitride ligands formed via N�N bond cleavage with various carbon-centered electrophiles has been intensively examined.
Peters and coworkers reported the conversion of the nitride ligand on the molybdenum complex 11, prepared via direct cleavage of N�N bond of N 2 , into an organic amide by the reaction with trifluoroacetic anhydride at room temperature (Scheme 10). [53]However, the amide ligand of the complex 11 decomposed to serve as a proton source in the reaction, thus preventing further application of this system into a synthetic or catalytic cycle.
In 2006, Cummins and coworkers reported a synthetic cycle for nitriles from N 2 including cleavage of N�N bond as a key step, by the stepwise functionalization of 11 (Scheme 11). [54]he cycle started with the reaction of 11 with Cummins reported another synthetic cycle for the formation of nitriles based on the heterobimetallic dinuclear complex (20, Scheme 12). [55]Reduction of the complex 20 with Na/Hg afforded the anionic niobium-nitride complex Na Direct cleavage of N�N bond of dinitrogen using molybdenum triamide complex. [45]heme 10.Amide synthesis by the reaction between acid anhydride and molybdenum-nitride complex synthesized via direct cleavage of N 2 . [53]heme 11.Synthetic cycles for nitriles using molybdenum triamide complexes. [54]omplex [Nb(OTf)(N(Np)(3,5-Xyl)) 3 ] (24), which served as a precursor for the heterodinuclear complex 20.
In addition to molybdenum and niobium complexes bearing triamide ligands developed by Cummins and coworkers, transition metal complexes bearing pincer-type ligands have also shown to undergo cleavage of N�N bond to give the corresponding terminal nitride complexes.In 2012, Schrock and coworkers found that the molybdenum diiodide complex bearing an anionic POCOP-type pincer ligand (25, POCOP = 2,6bis(di-tert-butylphosphinooxy)phenyl) can cleave N 2 upon reduction with Na/Hg to afford the corresponding anionic terminal nitride complex (26). [48]Mézailles and coworkers reported that the nitride complex 26 underwent nitride-alkyne cross metathesis reaction with internal alkynes to yield the corresponding nitriles.In this unique transformation, one electron oxidation of the intermediate with [Fc][BAr F 4 ] (Ar F = 3,5bis(trifluoromethyl)phenyl) plays a key role (Scheme 13). [56]chneider and coworkers reported several synthetic cycles to afford nitriles and/or amides based on acylation of nitride complex derived from N 2 by using transition metal complexes bearing amide-containing PNP-type pincer ligands.Rhenium dichloride complex bearing PN amide P tBu -pincer ligand (28, PN amide P tBu = bis(di-tert-butylphosphinoethyl)amide) can cleave N 2 triggered by reduction with Na/Hg or CoCp* 2 to give the corresponding rhenium-nitride complex (29), where a N 2bridged dinuclear complex was proposed as a reactive intermediate. [51]The resultant nitride complex 29 reacted with MeOTf as a carbon-centered electrophile to afford the alkylimide complex 30, while protonation of 29 with HOTf proceeded at the amide ligand to give the corresponding cationic nitride complex (31, Scheme 14).Based on this reactivity, Schneider and coworkers established synthetic cycles to afford acetonitrile (Scheme 15) [57] as well as benzonitrile from N 2 . [58]Complex 29 was ethylated with EtOTf to afford the ethylimide complex (32)  and subsequent deprotonation at the α-carbon in ethyl group gave the aldimide complex (33).Oxidation of 33 with Nchlorosuccinimide (NCS) afforded the rhenium trichloride complex 34 together with MeCN.Finally, reduction of 34 under N 2 atmosphere regenerated the starting nitride complex to complete the synthetic cycle.
Terminal nitride complexes formed via cleavage of N�N bond can react not only with highly reactive carbon-centered electrophiles such as acid chlorides and alkylation agents as described above, but also with poorly reactive ones featuring multiple bonds such as CO, CO 2 , and (hetero)cumulene. [13]In 2015, Sita and coworkers achieved molybdenum-or tungstenmediated synthetic cycle to give TMSNCO from N 2 , CO 2 and TMSCl (Scheme 18). [60]Silylation of N 2 -bridged dimolybdenum complex (40) under light irradiation gave the corresponding silylimide complex (41) and dichloro complex (42) via bis(μnitride-bridged) dimolybdenum complex generated via cleavage of N�N bond.The imide-oxo metathesis of 41 with CO 2 yielded TMSNCO and the corresponding oxo complex (43).Deoxygenation of 43 with TMSCl generated 42, then reduction of 42 under a N 2 atmosphere regenerated the starting complex 40.

Catalytic and Direct Formation of Organonitrogen Compounds from Dinitrogen
As shown in the previous sections, we overviewed the stoichiometric reactivity of N 2 complexes and nitride complexes derived from N 2 and their application to synthetic cycles for organonitrogen compounds.However, all of the synthetic cycles are only a combination of stoichiometric reactions.It should be well understood that it is quite difficult to achieve the catalytic reaction because of the low yield for each step and/or the use of incompatible reagents in the synthetic cycles.It is generally believed that the formation of nitrogen-carbon bonds is difficult and the use of highly reactive carbon-centered electrophiles that cause oxidative decomposition of the complexes is required.Furthermore, nitrogen-carbon bond containing moieties attached to the transition metal complexes are difficult to be liberated in many cases.Despite these many obstacles, we have achieved the first successful example of the catalytic and direct synthesis of an organonitrogen compound containing a nitrogen-carbon bond derived from N 2 with molybdenum-nitride complexes bearing pincer-type ligands as key intermediates.In this section, we introduce this interesting finding together with our research background.
As an extensive study of the development of catalytic nitrogen fixation using transition metal complexes, Nishibayashi and coworkers recently found the catalytic ammonia formation from N 2 under ambient reaction conditions using molybdenum triiodide complex bearing a pyridine-based PNP-type pincer ligand (15, Scheme 24a).In this reaction system, a molybdenum-nitride complex bearing a pyridine-based PNP-type pincer ligand ( 68) is formed via direct cleavage of bridged N 2 ligand, and subsequent formation of nitrogen-hydrogen bonds leads to the ammonia complex (69) which liberates free ammonia together with the nitride complex 68 (Scheme 24b). [52,67]More recently, Nishibayashi and coworkers found the use of SmI 2 as reductant and H 2 O as proton source led to the remarkable catalytic activity toward ammonia formation from N 2 under ambient reaction conditions, where up to 60000 equiv. of ammonia were produced based on the molybdenum atom of the catalysts with a molybdenum turnover frequency of up to 800 equiv.min À 1 . [8,68]Based on the highly efficient catalytic cycle for the formation of ammonia from N 2 , we have envisaged the formation of organonitrogen compounds from N 2 when carbon-centered electrophiles were used instead of proton source under the same reaction conditions.In order to achieve novel catalytic reactions for the formation of organonitrogen compounds, we first investigated stoichiometric reactions of molybdenum-nitride complexes in detail and then we tried to achieve the catalytic reactions based on information of the stoichiometric reactions.
We carried out stoichiometric reactions of molybdenumnitride complex bearing PNP-type pincer ligand 68, prepared by the reduction of the corresponding molybdenum triiodide complex 15 under atmospheric pressure of N 2 , with various carbon-centered electrophiles (Scheme 25). [69]eactions with MeOTf, 2,2-diphenylacetyl chloride, and phenyl chloroformate at room temperature gave the corresponding N-functionalized complexes such as methylimide, acylimide, and carbamate complexes (70-72) in good to high yields, respectively.On the other hand, reactions with phenyl isocyanate and diphenylketene under the same reaction conditions afforded the corresponding acylimide complexes bearing a second electrophile attached at the benzylic position of the pincer ligand (73 and 74).In sharp contrast, no formation of nitrogen-carbon bonds at the nitride ligand on 68 was observed when benzoic anhydride and carbon monoxide were used as carbon-centered electrophiles.In both cases, only ligand exchange of iodide ligand occurred to give the corresponding nitride complexes (75 and 76).
Based on the stoichiometric reactions shown in Scheme 25, we investigated further reactions of the N-functionalized Scheme 23.Synthetic cycle for TMSNCO by N 2 and CO 2 mediated by dititanium complex. [66]heme 24.Catalytic formation of ammonia via direct cleavage of N 2 using molybdenum complex bearing pincer type ligand. [52,67]omplexes.As a result, we observed an extremely unique reactivity of the carbamate complex 72.In fact, the reaction of the carbamate complex 72 with an excess amount of SmI 2 in THF at room temperature for 2 h gave the corresponding nitride complex 68 in 74 % NMR yield together with samarium species such as SmI 2 (OPh) in 93 % NMR yield and SmI 2 (NCO) in 75 % ion chromatography (IC) yield, respectively (Scheme 26a).The 15 N-labelling experiments indicate that the nitrogen atom in the isocyanate moiety was derived from the nitride ligand.The result of the two kinds of stoichiometric reactions under similar reaction conditions suggests that it is possible to achieve the synthetic cycle for the formation of cyanate anion (NCO À ) from N 2 catalyzed by the molybdenum complexes bearing the PNP-type pincer ligand (Scheme 26b).
When the atmospheric pressure of N 2 was reacted with 0.12 mmol of phenyl chloroformate (R = Ph) and 0.36 mmol of SmI 2 in the presence of 0.01 mmol of the molybdenum-nitride complex bearing PNP-type pincer ligand 68, only 0.45 equiv. of NCO À were formed based on the molybdenum atom of the catalyst.The use of the combination of methyl chloroformate (R = Me) and molybdenum-nitride complex bearing PCP-type pincer ligand [Mo(N)I(PCP)] (77, PCP = 1,3-bis(di-tertbutylphosphinomethyl)benzimidazol-2-ylidene), in place of phenyl chloroformate and 68, improved the amount of formed NCO À to 0.74 equiv.based on the molybdenum atom.After the detailed investigation, we finally found the catalytic reaction of N 2 with SmI 2 and chloroformates in the presence of a catalytic amount of molybdenum-nitride complex bearing a PCP-type pincer ligand 77 in THF under ambient reaction conditions to give up to 9 equiv. of NCO À based on the molybdenum atom of the catalyst in 75 % yield based on SmI 2 although slow addition of a THF solution of methyl chloroformate using syringe pump was necessary (Scheme 27).This is the first successful example of catalytic and direct transformation of N 2 into nitrogencarbon bond containing compound under ambient reaction conditions.
To get more information on the reaction pathway, we isolated the molybdenum-isocyanate complex [Mo-(NCO)ICl(PNP)] (78) from the reaction of the carbamate complex 72 with 1 equiv. of SmI 2 under 1 atm of Ar.Further reduction of the isocyanate complex 78 with an excess amount of SmI 2 under 1 atm of N 2 to afford the nitride complex 68 and NCO À in Scheme 25.Stoichiometric reaction of nitride complex bearing PNP-type pincer ligand formed via direct cleavage of N 2 with carbon-centered reactants. [10,69]heme 26.a) Reduction of the carbamate complex 72 with SmI 2 under N 2 to give the nitride complex 68.b) Two-step synthetic cycle to give cyanate anion (NCO À ) mediated by molybdenum complex. [10]6 % and 75 % yields, respectively.DFT calculations based on the stoichiometric reactions support a proposed reaction pathway shown in Scheme 28.Interestingly, when we carried out reduction of the carbamate complex 72 with 1 equiv. of CoCp* 2 under the same reaction conditions, another isocyanate complex [Mo(NCO)(OPh)Cl(PNP)] was obtained in 49 % yield.This experimental result indicates the importance of highly oxophilic samarium species to remove the phenoxy ligand from the isocyanate complex (Scheme 28).Thus, the key to achieve the catalytic reaction is the interconversion between the carbamate and isocyanate complexes, enabled by the combination of high oxophilicity of samarium compounds and high reactivity of chloroformate esters bearing an alkoxy group as a leaving group.

Summary and Outlook
In this short review, we summarized the previous examples and strategies for achieving direct transformation of N 2 into organonitrogen compounds.We demonstrated that the various transition metal N 2 complexes or nitride complexes derived from N 2 undergoes N-functionalization reaction with carbon sources, and that in some cases synthetic cycles were established based on these stoichiometric reactions.Finally, the first catalytic formation of nitrogen-carbon bond containing compound (NCO À ) from N 2 mediated by the molybdenum complex was introduced.This was achieved by the extension of the synthetic cycle for NCO À comprising two stoichiometric reactions (the fewest number of steps reported so far) that operates under similar and relatively mild conditions.NaNCO and KNCO are commercially important compounds applied in steel hardening, fine chemical synthesis, and the agrochemical industry with the world demand of 8000-10000 tonnes per year. [70]The direct and catalytic transformation of N 2 to useful nitrogencarbon bond containing compounds under mild condition is a significant step forward from stoichiometric reactions.However, some points still need improvement: necessity of slow addition by syringe pump and low turnover number, as well as low atom economy.For further improvement of the catalytic reaction, optimization of reaction conditions including other combinations of the reagents should be considered to avoid side reactions.Since reducing agents and highly reactive carboncentered electrophile coexisting in the current reaction system is generally incompatible with each other, exploration of milder reagents to selectively form organonitrogen compounds is required.It is also important to explore further about the reactivity of the metal complexes after nitrogenÀ carbon bond formation toward liberation of organonitrogen compounds from metal center.Compared to catalytic ammonia synthesis from N 2 , attempts for direct and catalytic synthesis of organonitrogen compounds from N 2 have just begun.We hope that this concept will contribute to the discovery of more efficient catalytic systems for the synthesis of organonitrogen compounds under mild reaction conditions. [71] 3 N as a base to form acyldiazenide complexes [W(NNCOR)(dppe) 2 Cl].Protonation of [W(NNCOR)(dppe) 2 Cl] ) by treatment with MeI at Shun Suginome received his PhD in 2022 from The University of Tokyo under the supervision of Professor Takuzo Aida, where he worked on the development of novel porous crystalline materials.He became an assistant professor in the group of Professor Yoshiaki Nishibayashi at The University of Tokyo in 2022.In the current lab, he has started research projects totally different to his PhD study, focusing on the synthesis of nitrogen containing compounds including organonitrogen compounds directly from dinitrogen based on transition metal complexes.Yoshiaki Nishibayashi received his PhD in 1995 from Kyoto University under the supervision of Professor Sakae Uemura.He became an assistant professor at The University of Tokyo in 1995 and moved to Kyoto University in 2000.In 2005, he became an associate professor at The University of Tokyo as PI.Since 2016, he has been a full professor at The University of Tokyo.His current research interests are organic and organometallic chemistry.